Magnesium oxide steel coating composition and process

ABSTRACT

Magnesium oxide coating compositions, process for their preparation and process for applying said compositions as adherent dried coatings which prevent fusing or welding of coiled steel sheet during an annealing process, and which when annealed provide relatively non-porous insulations on grain-oriented silicon-containing steel. The coating compositions comprise a major proportion of magnesium oxide sintered to a specific citric acid activity and pore volume and a minor proportion of: (I) a chloride contributor such as magnesium chloride, barium chloride or chromous chloride, and (II) sodium metasilicate. A significant feature of the present coating compositions is their resistance to excessive hydration when formed into aqueous slurries for coating onto steel. That is, the slurries do not hydrate to form appreciable quantities of magnesium hydroxide which is deleterious because it liberates water during high temperature annealing which has the effect of impairing the insulation.

Steger 1 1 Oct. 15, 1974 1 MAGNESIUM OXIDE STEEL COATING COMPOSlTION AND PROCESS [75] lnventor: John F. Steger, Crystal Lake, 111.

[73] Assignee: Morton-Norwich Products, 1nc.,

Chicago, 111.

22 Filed: Sept. 12,1973

2| App1.No.:396,346

[56] References Cited UNITED STATES PATENTS 3,265,600 8/1966 Carter 148/27 X 3,582,407 6/1971 Steger et al 3,583,887 6/1971 Steger et a1 148/27 Primary Examiner-Ralph S. Kendall ABSTRACT u yr -e o u ositigns, process for mdwfifibr applying said compositions as adherent dried coatings which prevent fusing or welding of coiled steel sheet during an annealing process, and which when annealed provide relatively non-porous insulations on grain-oriented silicon-containing steel. The coating compositions comprise a major proportion of magnesium oxide sintered to a specific citric acid activity and pore volume and a minor proportion of: (l) a chloride contributor such as magnesium chloride, barium chloride prchromous EhTorl n fliiitasilicar A i nificam feature of the present caiffiig comflsitions is their resistance to excessive hydration when formed into aqueous slurries for coating onto steel. That is, the slurries do not hydrate to form appreciable quantities of magnesium hydroxide which is deleterious because it liberates water during high temperature annealing which has the effect of impairing the insulation.

10 Claims, No Drawings MAGNESIUM OXIDE STEEL COATING COMPOSITION AND PROCESS BACKGROUND OF THE INVENTION 1. Field of the Invention As is described in U.S. Pat. No. 3,583,887, electrical transformers and other types of electrical induction apparatus are made using core materials made from steel sheet having soft magnetic properties. Such sheet is prepared by cold rolling silicon-containing steel into sheet form, coiling the steel sheet into rolls and thereafter annealing the coiled steel by a controlled heating process to produce a grain oriented structure having desirable magnetic properties.

Magnesium oxide is used extensively as a highly heat resistant separator medium and protective coating for metal surfaces. It is also used for forming an electrical insulator coating for metals, as a gatherer for removing impurities, such as sulfur and carbon, from thin metal sheets and particularly for forming a protective base insulation for silicon steel to which other coatings, e.g., phosphate coatings, may be added.

2. Description of the Prior Art Referring again to U.S. Pat. No. 3,583,887, according to the present industrial practice, silicon-containing steel is cold rolled into sheets, decarburized and thereafter coiled into convenient rolls. Cold rolling develops in the steel the potential to form a grain oriented structure when the steel is later annealed." The term annealed refers to a process whereby the steel is heated to about l,200C. in an atmosphere of low dewpoint containing hydrogen, or in a vacuum, under programmed conditions with respect to time and temperature. This results in a growth in size of the steel grains and also in a specific grain orientation which provides the desired soft magnetic properties sought. During the annealing process. virtually all of the remaining excess carbon and sulfur content of the steel is lost.

During the annealing process in hydrogen where the steel is in large coils, in the absence of a suitable separating medium, the coiled roll would fuse to itself and could not be unrolled. Conventionally, this is avoided by placing a thin coating of magnesium oxide on the steel prior to coiling. Further, the magnesium oxide coating serves to reduce impurities such as carbon and sulfur in steel by chemical reaction. In addition, magnesium oxide provides a major part of an electrically insulating silicate layer by reaction with the steel. For most applications, this silicate insulation is important to form an efficient electrical insulating coating or as the base coating upon which such insulation is formed. Thus, for example, transformer cores are constructed from thin sheets of soft magnetic steel stacked together to form a laminated body in which each sheet is electrically insulated from its neighbor. This construction vastly reduces the generation of eddy currents in the core imposed by an alternating electrical field. The average density of soft iron in the core should be as large as possible and consequently the insulation on the plates should be as thin as possible to provide closer stacking of steel plates.

Obviously, short circuits between plates reduces transformer efficiency and often causes the development of damaging hot spots in the transformer core. Consequently soft magnetic steel is rated by the number of short circuits per unit area, usually expressed in terms of the electrical resistance of the insulating layer.

This is a standard ASTM measurement known as the Franklin Test and is a measure of the conductance.

Various additives to magnesium oxide and processing techniques have been proposed to alleviate some of the deficiencies inherent in the use of magnesium oxide per se as a coating for steel. For example, as set forth in U.S. Pat. No. 2,385,332, magnesium oxide during heat treatment at suitable temperatures is reacted with silicon present in the steel to form a glass-like coating that is useful as an interlaminar base insulation for silicon iron electrical apparatus. transformer cores, etc. In addition, the following tabulation sets forth the additives and/or methods which have been proposed by the indicated patents to improve the performance of magnesium oxide:

U.S. Patent 2.809.137 Silica additive 2.394.047 Oxidation of steel surface 3.583.887 Oxidation with 5'10, and flux with boron compounds 3.697.322 Activation of MgO with lithium compounds German Patent 2.062.290

Serpentine additive For steel containing aluminum nitride, special additives have been proposed which facilitate both the formation of insulation and also proper grain growth for improved magnetic properties. U.S. Pat. No. 3,627,594 discloses air oxidation of this type of steel followed by coating with magnesium oxide plus titanium dioxide and manganese oxides. U.S. Pat. No. 3,676,227 discloses the use of magnesium oxide plus boron compounds.

However, a further problem arises because magnesium oxide is applied to steel from an aqueous suspension or slurry. In the presence of water, magnesium oxide hydrates to magnesium hydroxide to a substantial degree. The degree of hydration depends on a number of factors, such as, the surface area of the magnesium oxide, the temperature of the water and the residence time of the magnesium oxide in the water. It is known that the presence of magnesium hydroxide to any substantial degree in a magnesium oxide composition releases H O and, unless removed prior to annealing, can impair electrical properties of the dried and annealed steel coating. The term coating as used herein, means the dried magnesium oxide composition formed on the steel surface. The term insulation, as used herein, means that glassy composition formed when the coating is annealed. In an ordinary steel coating operation, it is therefore important to keep the aqueous residence time of the magnesium oxide composition to a predetermined minimum level. The steel is frequently coated while still hot from decarburization. Elevated temperatures imparted to the coating bath accelerate the rate of hydration. For high activity magnesium oxide, the residence time may be about 10 minutes, whereas for low activity magnesia the residence time may be about 30 minutes. This places a burden on a steel coating operation in that the aqueous slurry of magnesium oxide must be used within this relatively short span of time or else become impaired due to excessive hydration. It would therefore be desirable to attain the objective of providing magnesium oxide compositions, a process for their manufacture and a process for forming adherent coatings and electrically insulat- 3 ing coatings (insulations) for silicon-containing steel by the use of said compositions, all of which provide the following multiple benefits: 1

1. Aqueous slurries of the composition can be subjected to a wide range of hydration conditions without altering the quality of the coated and annealed steel. That is, maintaining the magnesia composition in an aqueous slurry at temperatures as high as 130F. for as long as 2 hours will not cause substantial hydration resulting in the formation of excessive deleterious magnesium hydroxide.

2. Coatings formed from the present compositions exhibit high density-and adherence to steel.

3. The dried and annealed steel insulations display good electrical insulating properties.

4. 1n the annealing operation, the coatings of the present invention do not give rise to offensive or corrosive fumes.

The achievement of the foregoing benefits is accomplished by the present invention and may be more readily appreciated by reference to the following specification, examples and appended claims.

SUMMARY OF THE INVENTION Broadly, the present invention provides a magnesium oxide composition for coating silicon-containing steel sheet to provide the aforesaid benefits, said composition comprising a major proportion of a sintered magnesia having a citric acid activity of from about 30 to 85 seconds and a pore volume of from about 0.02 to about 0.1 cc. per gram, and based on said magnesium oxide:

l. a chloride contributor selected from the group consisting of magnesium chloride, barium chloride and chromous chloride, said chloride contributor providing from about 4 X to about 60 X 10' moles of chloride ion per square centimeter of steel surface; and

ll. sodium metasilicate in a concentration at least about stoichiometrically equal to said chloride contributor.

This invention also provides a process for preparing said magnesium oxide composition for coating onto silicon-containing steel, said process comprising admixing sintered magnesium oxide with sodium metasilicate and a chloride contributor selected from the group consisting of magnesium chloride, barium chloride and chromous chloride, said sintered magnesium oxide having a citric acid activity of from about 30 to about 85 seconds and a pore volume of from about 0.02 to about 0.1 cc./gram, the concentration of said chloride contributor being such as to provide from about 4 X 10' to about 60 X 10" moles of chloride ion per square centimeter of steel surface and the concentration of sodium metasilicate being at least about stoichiometrically equal to said chloride concentration.

In one preferred embodiment, the magnesium oxide steel-coating composition of this invention is conveniently prepared as an aqueous slurry by adding said sintered magnesia to water containing sodium metasilicate and the chloride contributor. If desired, the sodium metasilicate may be combined with the sintered magnesia to form a homogeneous mass, and then added to water containing the chloride contributor to provide an aqueous slurry for use in a steel-coating operation.

Although any sintering agent which will aid in providing the specially sintered magnesium oxide of this invention may be employed. included among those which may be used are lithium chloride, boric oxides, boric acids, and magnesium salts of boric acids. By the term boric oxides", it is meant to include the oxides of said boric acids. By the term magnesium salts of boric acids, it is meant to include the various known magnesium borates. such as for example, the magnesium orthoborates, the magnesium metaborates and the magnesium pyroborates.

The present invention also provides a process for coating silicon-containing steel sheet with an adherent. electrically insulating coating of said magnesium oxide composition, which process comprises in sequence forming an aqueous slurry or suspension of said magnesium oxide composition, coating said slurry to the surface of said steel sheet, heating to remove water therefrom and dry the coating thereon, and thereafter annealing the dried coated steel sheet at a temperature in excess of about 1,000C.

Citric Acid Activity The citric acid activity is a measured of the activity of magnesium oxide and is determined by a method which measures the time required for a given weight of a particular magnesia to provide hydroxyl ions sufficient to neutralize a given weight of citric acid. The test is conducted as follows:

1. ml. of 0.400 normal aqueous citric acid containing 2 ml. of 1 percent phenophthalein indicator is brought to 30C. in an 8 ounce wide mouth jar. The jar is fitted with a screw cap and a magnetic stirrer bar.

2. Magnesia weighing 2.00 g. is admitted to the jar and a stopwatch is started at the same instant.

3. As soon as the magnesia sample is added the lid is screwed on the jar. At the 5 second point the jar and contents are vigorously shaken. Shaking is terminated at the l0-second point.

4. At the 10-second point the sample is placed on a magnetic stirrer assembly. Mechanical stirring should produce a vortex about 2 centimeters deep at the center when the inside diameter of the jar is 6 cm.

5. The stopwatch is stopped the instant the suspension turns pink and the time is noted. This time in seconds is the citric acid activity.

Pore Volume This is here defined by the following relationship:

(g. sodium metasilicate)/(g. MgO X 1.52 g./cc.)

in which 1.52 is the density of molten sodium metasilicate. Pore volume, as used herein, is taken to be that value (from the above expression) at which the sintered magnesium oxide displays good adherence to steel, as determined by the use of sodium metasilicate in conjunction with the MgO. The procedure for determining pore volume is as follows:

l. Coat steel strips with slurries containing a mixture of MgO and Na SiO 2. Dry the strips in a flow of air at 1.88 ft./sec. and at a temperature of 1,300F., exposing the strip to these conditions for 15 seconds. The slurry concentration should be chosen so that the coating weight is less than about 0.08 ounces per square foot of steel.

3. Determine the adherence of the coating by gently rubbing the pad of the index finger over the coating. Adherence is evaluated as:

Result Adherence Rating All MgO rubs off Poor Some MgO rubs off leaving bare steel Fair Some MgO rubs off leaving coated steel Good No MgO rubs off Excellent 4. Progressively increase the Na SiO IMgO ratio and repeat steps I to 3 until the adherence is excellent.

5. Divide the g./g. of Na siO lMgO that produces excellent adherence by 1.52 to compute the pore volume in cc./g.

it is believed that the chloride contributor, in conjunction with sodium metasilicate, functions to promote the formation of a nonporous insulation coating. The porosity of the coating formed by the present invention is determined by means of the Copper Plating Test which is hereinafter more fully described. However, in general, a porous coating will permit copper from a copper sulfate solution to deposit on the steel substrate whereas a nonporous coating will not. The concentration of chloride contributor employed is that which will provide from about 4 X to about 60 X 10 moles of chloride ion per square centimeter of steel when the present magnesium oxide composition is coated onto steel. The preferred chloride contributor is magnesium chloride. Magnesium chloride may be admixed directly with the sintered magnesia, however, it is preferred to add it to the water used to form the aqueous coating slurry.

The quantity of sodium metasilicate employed is that which is at least stoichiometrically equal to the quantity of chloride contributor. in view of the reaction of the chloride contibutor. e.g., magnesium chloride, with sodium metasilicate to form magnesium silicate and sodium chloride in situ, and in view of the disclosure of US. Pat. No. 3,265,600 which teaches that an MgO coating containing sodium chloride provides poor insulation, it was surprising to find that the presence of sodium chloride in the present composition does not impair the electrical insulating properties of the dried and annealed coating.

For a steel coating operation, it is preferred that sintered magnesia be pulverized to a fineness such that less than about 0.5 percent by weight remains on a 325 mesh sieve when subjected to the following screening test:

Screen Test 1. Place a 3-inch diameter 325 mesh screen in a 6% inch diameter evaporating dish.

2. Add sufficient anhydrous denatured alcohol to attain a liquid level to one-half the height of the screen framework. The mesh should now be below the alcohol level, if not, add more alcohol.

3. Place 5 grams of sample on the screen, i.e., on the liquid within the framework of the screen.

4. Without raising the screen from the alcohol, brush the powder gently with a camels hair brush. The powder should become wet and pass through the screen.

5. After the powder has been wetted, the screen can be moved about and raised from the liquid as desired.

6. After the screening is judged complete, the screen is then rinsed with an alcohol stream from a wash bottle.

7. Air dry briefly, then place screen in oven 105C.) for l5 minutes.

8. Transfer material from the screen to a pre-weighed pan and weigh on analytical balance.

9. Calculate percent retained on 325 mesh as follows:

0 (Amount retained on 325 mesh screen) (lOO)/Sample Size The sodium metasilicate and chloride contributor may be blended with the pulverized sintered magnesia to form a homogeneous mass, or it may be added to the water used to form the aqueous coating slurry. It appears to be a simpler procedure to add both the sodium metasilicate and the magnesium chloride directly to the water rather than to attempt to form a homogeneous dry blend with the sintered magnesia.

An important advantage of the present composition is its resistance to excessive hydration when formed into an aqueous slurry. By excessive hydration, it is meant that degree of hydration which results in the formation of red or black iron oxides visible to the naked eye on the surface of the steel. When ordinary magnesias are placed in water at temperatures up to about F., they hydrate within about 30 minutes to form substantial quantities of magnesium hydroxide. Under similar conditions, the present magnesia compositions are resistant to excessive hydration for as long as two hours. it is recognized in the art that excessive quantities of magnesium hydroxide in a magnesia impairs the electrical insulating properties of that magnesia when coated onto steel plate and thereafter annealed. The advantage gained with the present hydration resistant magnesia compositions is that they permit a substantially longer residence time in an aqueous slurry before hydrating to form quantities of magnesium hydroxide which in excessive quantities are deleterious to the electrical insulating properties of the ultimate insulation. This feature is an added safety margin in a steel coating operation where, under plant conditions, it is sometimes not possible to complete the steel coating procedure using an aqueous slurry in less than about 30 minutes. Nor is it always possible to completely cool the steel from the decarburization line.

For coating steel, an aqueous suspension of the above described magnesium oxide composition is prepared by mixing with water to the desired viscosity and leveling and flow-out characteristics. Generally from about 5 to about 20 weight percent of the magnesium oxide composition, based on water, is satisfactory to provide an aqueous slurry having the requisite viscosity and flow properties suitable for coating onto steel sheet.

The coating slurry may be applied to the magnetic sheet material by any suitable meanssuch as by immersion, brushing, or spraying. it has been found convenient to use an immersion technique whereby the steel sheet is passed through a tank containing the coating slurry. The coated sheet thereafter is heated to drive off water and provide a dried layer of the present composition. The coated metal sheet is then coiled into a roll and placed in a furnace for annealing as previously described. During the annealing process, the coating of this invention forms an adherent, electrically insulating, corrosion-resistant layer which also functions as a separator medium to prevent the coiled metal sheet from sticking to itself.

As more fully described in the following examples, the present magnesium oxide coating compositions were coated onto steel sheets having a width of 3 centimeters.

For a more complete understanding of the present invention, reference is now made to the following specific examples illustrating the improved properties of the coating compositions of this invention and of the process for preparing said compositions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 A steel coating slurry or suspension was prepared from a commercially available magnesium oxide containing no additives and having a Citric Acid Activity of 19.5 to provide a slurry containing 7.2% MgO by weight. The resultant suspension was maintained at a temperature of 1 10F. for from about 25 to 50 minutes under constant stirring and was then coated onto a number of silicon steel sheets prepared from the same steel melt, each sheet measuring 3.0 centimeters in width and 12-14 mils in thickness. The coatings were leveled and then dried to a steel temperature of about 205C. reached in 15 seconds, to provide a dried coating weight of from about 0.03 to about 0.035 ounces per square foot of steel, the usual coating weight employed in the steel industry.

The Adherence of the dried coating and its Loss on Ignition were determined according to the procedures hereinafter described.

The dried coated steel sheets were annealed by heating at l.l77C. for a fixed time interval. The annealed sheets were then cooled, and excess or loose magnesium oxide was scrubbed from the surface of each coated sheet by brushing in a stream of flowing water.

The Conductance of the coated and annealed surface was measured by means of the Franklin Test, and the Porosity of this surface was determined by means of the Copper Plating Test.

Description of Tests ADI-IERENCE TEST The Adherence of the dried coating is determined by gently rubbing the pad of the index finger over the coating as previously described and evaluated.

LOSS ON IGNITION The specimen is heated at l,0OOC. for one hour. The Loss on Ignition is a measure of the degree of hydration of MgO, i.e., of the formation of Mg(OH),.

CONDUCT ANCE (FRANKLIN TEST) This test is widely accepted and utilized for evaluating the conductance of coated steel sheets. A detailed description is found in ASTM method A-334-52, Standard Methods of Test for Electrical and Mechanical Properties of Magnetic Materials. Briefly, the test is carried out by passing an electric current through brass contacts which cover coated areas 0.1 square inch in area. Current passing through the coating flows through the steel to a contact made directly to the steel by means of a twist drill. The resulting amperage provides a measure of the resistance encountered through the coating. Several hundred contacts are employed in obtaining readings for coating evaluation. A complete short circuit, i.e.. complete Conductance, is indicated by reading of I00 milliamps per 0.1 square inch. Therefore, the lower the reading in milliamps the more insulating the coating. In the present invention, Franklin values of about 50 or less are considered acceptable.

POROSITY (COPPER PLATING TEST) A steel sheet bearing a dried and annealed coating is immersed in an aqueous solution of copper sulfate. Copper spontaneously plates on the surfaces that are not electrically insulated, and therefore provides an indication of the Porosity of the annealed coating.

l. lhe Reagent g.lliter H 0 of CuSO..5H=O ll. Procedure A. Fill a 200 ml. tall form beaker with about I00 ml. of copper sulfate solution. B. With the solution at about 80F. partially immerse the sample of coated and annealed steel for 30 seconds moving the sample with a fanning motion. After 30 seconds. wash off adhering reagent by holding the strip under a stream of tap water. Wipe the strip and allow it to dry. The reagent can be reused a number of times without grossly affecting the results. It should be discarded when it visually appears to contain too much sulfate. lntegpretation of Results Copper will plate on many types of defects that are not directly produced as a result of the magnesia or the steel quality. Hence edge effects from annealing small strips. accidental mechanical or surface marking are discounted for such purposes. Evaluations of magnesia, steel. and processing quality are accordingly based on the appearance of overall centrally located areas. The extent of plating can vary from complete to no plating at all. The results are characterized as follows:

Ill.

Result Porosity Completely free of copper plating Good Traces of or partially copper plated Fair Completely copper plated Bad The procedure of Example IA was repeated except that the magnesium oxide employed was a sintered MgO containing 0.05 percent by weight of lithium chloride and having a citric acid activity of 50 and a pore volume of 0.05 cc. per gram. An 1 1.8 percent suspension of the magnesia in water was prepared containing (on an MgO basis) 0.5 percent magnesium chloride and 1.7 percent by weight of sodium silicate. The suspension so forrned was maintained at a temperature of F. for 60 minutes before coating onto the steel sheets, and then dried so that the steel reached 205C. in 15 seconds. The resultant dried and annealed coating therefore was formed from the following constituents:

MgO

- Citric acid activity 50 Pore volume 0.05 cc. per gram Lithium chloride 0.05% Magnesium chloride 0.5% Sodium silicate l.7%

The Adherence, Loss in Ignition, Conductance and Porosity tests were performed as in Example lA. 1C

The procedure of Example 18 was repeated except that magnesium chloride was not added in forming the steel coating suspension. The Adherence, Loss on Ignition, Conductance and Porosity tests were performed as in Example 1A.

The results of the foregoing tests are summarized in Table l as follows:

EXAMPLE 2 10 insulation coating was determined to be good. This not 1 Conductance Hydration Loss on (Franklin Example Conditions Ignition Adherence Moles Cl per CmF/Steel Values) Porosity IA -50 Min. 7-] 1% Good-Excellent 62 Good 1B 60 Min. 3% Excellent 4.4 X IO" to 13.1 X lO'" 20-35 Good IF. Not

IC 60 Min. Determined Excellent 4043 Bad Range for several determinations demonstrates that the present invention i s operable for all types of decarburized steel.

EXAMPLE 3 The procedure of Example 1B was repeated except that the concentration of magnesium chloride was varied to provide from 4 X ID to 158 X 10'" moles chloride per square centimeter of steel and the coating weights varied from 0.047 to 0.066 ounces per square foot of steel. The results are set forth in Table 2.

' Table Coating Wt. Moles Cl. Franklin OzJft. Adherence Per cm. Steel Value Porosity 0.047 Excellent 4 X lO' 28 Good 0.06l Excellent ll X 10" 20 Good 0.065 Excellent 22 X lo 20 Good 0.050 Excellent 32 X I0" 52 Poor 0.050 Good-Excellent 64 X lO 30 Poor 0.052 Good-Excellent 158 X 10" 58 Poor Calculated from the expression: [Moles Cl from LiCl/g. MgO Moles Cl from MgCl,/g. MgO] X g. MgO/Crn. Steel EXAMPLE 4 The procedure of Example 18 was repeated except that barium chloride was substituted for magnesium chloride, the coating weights were varied from 0.067 to 2. There is a significant improvement (decrease) in Conductance of the dried and annealed coating of Example 18 when compared with that of Example I l A. This is further illustrated by the following tabulmmn: m mm m in 0.085 ounces per square foot of steel, and the concen- Eillllplf Conductance (inductance tration of barium chloride was was varied to provide from 6 X 10 to 176 X 10' moles of chloride per lA o2 62/20X |00=3|01 B 2045 (62/35 X m0 177%) square centimeter of steel. The results are set forth in Table 3.

' Table 3 Coating Wt. Moles Cl. Franklin Oz./ft. Adherence Per Cm. Steel Value Porosity 0.067 Good-Excellent 6 X l ll Poor 0.067 do. 10 x 10* 6 Good 0085 do. 27 x 10-" 17 Good 0.085 do. 45 x 10- 23 Poor 0.070 do. 72 x 10- 49 Poor 0.069 do. 176 x 10" Poor Calculated from the expression: [Moles Cl from LiCl/g. MgO Moles Cl from Bach/g. MgO] x g. MgO/Cm. Steel EXAMPLE 5 The procedure of Example 13 was repeated except that chromous chloride was substituted for magnesium chloride, the coating weights were varied from 0.041 to 0.074 ounces per square foot of steel, and the concentration of chromous chloride was varied to provide from 3 X to 120 X 10- moles of chloride per square centimeter of steel. The results are set forth in EXAMPLE 7 The procedure of Example 18 was repeated except that the weight proportions of Na SiO relative to MgCl were varied from 3.4:] .0 to 0.43:1.0 to ascertain the concentration of Na SiO required with respect to MgCl to provide dried, annealed coatings having good insulation (i.e., a Franklin value less than 50) and good la b le A. ,7 Y A l0 porosity. The results are set forth in Table 6.

Table 4 Coating Wt. Moles Cl" Franklin OzJFt. Adherence Per Cm. Steel Value Porosity 0.041 Good-Excellent 3 X 10" 67 Poor 0.054 Excellent 7 X 10" 20 Fair 0.074 Excellent 17 X 10" Good 0.052 Excellent 23 X 10" 70 Fair-Poor 0.050 Excellent 44 X 10" 79 Poor 0056 Excellent 120 X 10" 85 Poor Calculated from the expression: [Moles Cl from LiCl/g. MgO Moles Cl from CrCl,/g. MgOl X g.

MgO/Cm. Steel The ltoichiornetric ratio of Na,SiO, 2 MgCl, with respect to the formation of magnesium silicate and sodium chloride is 1.24:1.0

EXAMPLE 6 The foregoing demonstrates that a quantity of sodium silicate substantially less than that stoichiometrically required to react with the magnesium chloride provides coatings which are deficient with respect to insulation (Franklin value) and/or porosity.

EXAMPLE 8 The procedure of Example 1B was repeated except that the proportions of magnesium chloride and sodium silicate, the coating weights and the number of moles of chloride per square centimeter of steel surface were varied as indicated in Table 5 with the results as set forth therein. The rheology or flow characteristics of tl1e coa ti ng slurries were also evaluated.

The procedure of Example 1B was repeated except lL L Efi l i 2 292093392521??? wei intq sd Table 5 Slurry Franklin 1: MgCl, k Nit SiO; Rheology Coating Wt. Oz./Ft. Adherence Moles Cl per Cm.= Value Porosity 0.5 L7 Good 0.07l0.077 Good-Excellent 13 X IU 31-37 Good 0.125 0.425 Good 0.071 Excellent 4 X 10" 25 Fair 0.06 0.2 Poor 0.050 Good 2 X l0" 42 Fair 0.03 0.1 Poor" 0.050 Good 1 X 10' 63 Poor The slurry was thin", i.e., low viscosity. and therefore unsuitable for coating.

13 MgO prepared by sintering basic magnesium carbonate with 0.05 percent by weight boric acid, and having a citric acid activity of 55 and a pore volume of 0.05 cc.

l4 MgO were added to water containing 1.7 percent Na SiO (MgO basis) and commensurate concentrations of MgCl to provide slurries which when coated onto steel per gram. yielded the following coating weights and moles of Cl 88 5 per Cm:

The procedure of Example 8A was repeated except that 1.0 percent by weight of magnesium chloride was used instead of 0.5 percent. Coating Weight Moles Cl 8C Example OzJFt. Per Cm.

The procedure of Example 8A was repeated except 10 9A 0.077 13.7 x 10* that no magnesium chloride was used. 95 0953 95 X 9C 0.046 8.2 10 In Examples 8A, 8B. and 8C. the steel used for coat- 9D (1033 X -t ing was a regular grain oriented type steel. The results 3? 8-8:: 3-: X obtained were as follows: X

Coating Wt. Moles Cl Franklin Example Oz./Square Ft. Adherence Per Cm. Value Porosity Fair 8A 0.03 Good 4.8 x 27 to Good 88 0.07 Good 22.4 x 10-" I9 Fair Excellent sc 0.04 Fair 0 72 Bad The foregoing results demonstrate the value of MgCl in the present invention. 8D

To demonstrate the usefulness of the present composition in coating a specialty steel, namely, HB" steel which contains aluminum nitride and provides exceptionally high magnetic permeability, the following pro cedure was employed:

An aqueous slurry was prepared containing 24 grams of a sintered magnesia having a citric acid activity of 50 seconds and a pore volume of 0.038 cc./gram, 0.05 percent of lithium chloride, 0.5 percent magnesium chloride and 1.7 percent sodium metasilicate. This slurrry was held (hydrated) at a temperature of 130F. for 60 The Adherence and the texture and quality of the coatings was determined. The coating texture was evaluated as follows:

The results obtained are set forth in Table 7.

Table 7 Coating Wt. Coating Moles Cl Example O2./Ft. Texture Per Cm. Adherence 9A 0.077 Good 13.7 X 10" Good-Excellent 98 0.053 Good 9.5 X 10-" Good 9C 0.046 Good 8.2 X l0 Fair-Good 9D 0.033 Fair 5.9 X IO" Fair 9E 0.019 Poor 3 4 X IO' Poor 9F 0.018 Poor 3 4 X 10" Poor minutes and then coated onto HB steel sheets. The EXAMPLE l0 coated sheets were dried at 1,300F. for 15 seconds and then annealed at l,l77C. The results were as follows: p

The procedure of Example 18 was repeated with respect to the provision of dried cotaings in a series of experiments in which different percentages of sintered A series of experiments was conducted in which therocedure of Example 18 was repeated except that the following parameters were varied as indicated:

1. The coating slurry temperature was maintained at 130F. for a period of time ranging from 30 to minutes. This period of time is the hydration time.

2. The moles of chloride per square centimeter of steel ranged from about 10 X 10* to about 15 X 10.

The following properties of the coatings and of the 65 insulations so prepared were evaluated to determine the optimum drying conditions to provide the best annealed coatings according to the process of this invention:

Drying furnace temperature Temperature of steel after drying Texture of dried coating Adherence Loss on lgnition Moles Cl per Cm.

Color of steel oxide layer under dried coating Conductance (Franklin Value) Porosity l6 EXAMPLE 12A A steel coating slurry was prepared from a sintered commercially available magnesia having citric acid activity of 42 and a pore volume of 0.026 cc./gram by adding 24 grams of this magnesia to 180 milliliters of water containing 0.25 gram of magnesium chloride and 0.96 gram of sodium metasilicate. The resultant slurry was maintained at a temperature of l30F. for about 20 minutes under constant stirring and then coated onto The results are summarized in Table 8. t the same steel sheets as were employed in Example 1.

Table 8 Temp. Temp.

of of Steel Color Of Drying After Coating Wt. Loss on Oxide Layer Franklin Example Furnace Drying Oz./Ft. Texture Adherence Ignition Under Coating Value Porosity 10A l600 300 Good Excellent Black 52 Bad lOB l500 300 Good Excellent Reddish Black 44 Bad WC 1400 250 007-008 Good Good Excellent 3.2 Light Brown 36 Fair lOD 1300 205 0.06-0.08 Good Good-Excellent 4 Blue -37 Good 10E I200 I50 0.07-008 Good Excellent 6.7 Lavender 34 Fair l0F l 100 l l0 0.065 Good Excellent 4.8 Tan 29 Fair WC 1000 I00 0082 Good Excellent 9.5 Tan 10H 900 I00 0.082 Good Excellent 12.9 Tan A loss on ignition in excess of about 87? is considered evidence of excessive hydration.

EXAMPLE 11 The hydration resistance of the MgO coating composition of the present invention when compared to a commercially available MgO containing no additives, was determined by the following procedure:

A steel coating was prepared from a 10 percent slurry of a sintered MgO containing 0.05 percent by weight of LiCl and having citric acid activity of 50 and a pore volume of 0.05 cc. per gram. Also added to the water were 0.5 percent by weight of MgCl and 1.7 percent by weight of Na SiO based on the MgO. The resultant slurry was stirred continuously under standard conditions and maintained at a temperature of 130F. for 30 minutes. Thereafter it was coated onto the same steel sheets as used in Example 1A and in the same manner. The coating was removed and its Loss on Ignition at 1,000C. was determined as a measure of its hydration. As a control, a commercially available MgO containing no additives was carried through the same procedure.

The procedure of Example 1 1A was repeated except that the time of hydration, i.e., the time that the coating slurry was maintained at 130F. before coating, was 60 minutes. As a control, a commercially available MgO containing no additives was carried through the same procedure.

The coatings were leveled and then dried to a steel temperature of about 205C. reached in 15 seconds. The dried coated steel sheets were annealed by heating at l,l77C. for a fixed time interval. The annealed sheets were then cooled, and excess or loose magnesia was scrubbed from the surface of each coated sheet by brushing in a stream of flowing water. The Conductance. Adherence and Porosity were determined as hereinbefore described.

EXAMPLE 123 The procedure of Example 12A was repeated except that the magnesia employed had a citric acid activity of 39 and a pore volume of 0.053 cc./gram. Thirty-five grams of this magnesia were added to 180 milliliters of water containing 0.73 gram of magnesium chloride and 2.80 grams of sodium metasilicate to form the steelcoating suspension. The Conductance, Adherence and Porosity were determined as hereinbefore described.

Citric Moles Cl Conductance Acid Pore per (Franklin Ex. Activity Vol. emf/steel Value) Adherence Porosity 12A 42 0.026 l0 x 10-" 18 Good Excellent l2B 39 0.053 39 X l0" l4 Excellent Excellent 12C 84 0.073 58 x [0" 7 Good Excellent The results are summarized as follows: The foregoing results demonstrate that the concenf y L I U tration of chloride contributor (magnesium chloride) Example .ll l s lrE lrlills (P l' c rl t 15; 0/3?) 65 can be as high as about 60 X 10" moles per square centlmeter of steel in the present invention. NA 30 3.9 Control 30 l3.l

I I8 60 What is'EE'i'nicTTz Control 60 L 1. A magnesium oxide composition for coating sil con-containing steel sheet, said composition comprising a major proportion of a sintered magnesia having a citric acid activity of from about 30 to about 85 seconds and a pore volume of from about 0.02 to about 0.1 cc. per gram, and based on said magnesium oxide:

l. a chloride contributor selected from the group consisting of magnesium chloride. barium chloride and chromous chloride, said chloride contributor providing from about 4 X to about 60 X 10'" moles of chloride ion per square centimeter of steel chloride, barium chloride and chromous chloride, said sintered magnesium oxide having a citric acid activity of from about 30 to about seconds and a pore volume of from about 0.02 to about 0.1 cc./gram, the concentration of said chloride contributor being such as to provide from about 4 X IO to about 60 X lO moles of chloride ion per square centimeter of steel surface and the concentration of sodium metasilicate being at least about stoichiometrically equal to said chloride concentration.

6. The process of claim 5 wherein the chloride contributor is magnesium chloride.

7. The process of claim 5 wherein the chloride con tributor is barium chloride.

8. The process of claim 5 wherein the chloride contributor is chromous chloride.

9. A process for coating silicon-containing steel sheet with an electrically insulating coating which comprises in sequence forming an aqueous slurry of a magnesium oxide composition as defined in claim 1, coating said slurry to the surfaces of steel sheet, heating the coated steel to dry said coating thereon and thereafter annealing said dried coated steel at a temperature in excess of about l,0OOC.

10. A coated silicon-steel sheet having thereon an adhered coating of the composition defined in claim 1. 

1. A CHLORIDE CONTRIBUTOR SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM CHLORIDE, BARIUM CHLORIDE AND CHROMOUS CHLORIDE, SAID CHLORIDE CONTRIBUTOR PROVIDING FROM ABOUT 4 X 10**8 TO ABOUT 60 x 10**8 MOLES OF CHLORIDE ION PER SQUARE CENTIMETER OF STEEL SURFACE; AND II. SODIUM METASILICATE IN A CONCENTRATION AT LEAST ABOUT STOICHIOMETRICALLY EQUAL TO SAID CHLORIDE CONTRIBUTOR.
 1. A MAGNESIUM OXIDE COMPOSITION FOR COATING SILICONCONTAINING STEEL SHEET, SAID COMPOSITION COMPRISING A MAJOR PROPORTION OF ASINTERED MAGNESIA HAVING A CITRIC ACID ACTIVITY OF FROM ABOUT 30 TO ABOUT 85 SECONDS AND A PORE VOLUME OF FROM ABOUT 0.02 TO ABOUT 0.1 CC. PER GRAM, AND BASED ON SAID MAGNESIUM OXIDE::
 2. The composition of claim 1 wherein the chloride contributor is magnesium chloride.
 3. The composition of claim 1 wherein the chloride contributor is barium chloride.
 4. The composition of claim 1 wherein the chloride contributor is chromous chloride.
 5. A process for preparing a magnesium oxide composition for coating silicon-containing steel sheet, said process comprising admixing sintered magnesium oxide with sodium metasilicate and a chloride contributor selected from the group consisting of magnesium chloride, barium chloride and chromous chloride, said sintered magnesium oxide having a citric acid activity of from about 30 to about 85 seconds and a pore volume of from about 0.02 to about 0.1 cc./gram, the concentration of said chloride contributor being such as to provide from about 4 X 10 8 to about 60 X 10 8 moles of chloride ion per square centimeter of steel surface and the concentration of sodium metasilicate being at least about stoichiometrically equal to said chloride concentration.
 6. The process of claim 5 wherein the chloride contributor is magnesium chloride.
 7. The process of claim 5 wherein the chloride contributor is barium chloride.
 8. The process of claim 5 wherein the chloride contributor is chromous chloride.
 9. A process for coating silicon-containing steel sheet with an electrically insulating coating which comprises in sequence forming an aqueous slurry of a magnesium oxide composition as defined in claim 1, coating said slurry to the surfaces of steel sheet, heating the coated steel to dry said coating thereon and thereafter annealing said dried coated steel at a temperature in excess of about 1,000*C.
 10. A coated silicon-steel sheet having thereon an adhered coating of the composition defined in claim
 1. 